|Publication number||US4206401 A|
|Application number||US 05/896,070|
|Publication date||Jun 3, 1980|
|Filing date||Apr 13, 1978|
|Priority date||Apr 20, 1977|
|Publication number||05896070, 896070, US 4206401 A, US 4206401A, US-A-4206401, US4206401 A, US4206401A|
|Inventors||Hans U. Meyer|
|Original Assignee||Meyer Hans Ulrich|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Referenced by (22), Classifications (8)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention relates to a length-measuring system comprising two capacitors and electronic means in order to obtain an indication voltage precisely proportional to a length displacement.
Known capacitive length-measuring systems can be subdivided into two categories. The first category consists of length-measuring systems whose transducers comprise a variable capacitor. Such transducers have a very simple construction, but are however very sensitive to changes in the dielectric. The second category consists of length-measuring systems whose transducers comprise a differential capacitor and are consequently independent of changes to the dielectric. Transducers of the second type also allow compensation measuring methods, which significantly increase the measuring precision and lack of sensitivity to external influences. In a known method (U.S. Pat. No. 3,857,092) the transducer a.c. voltages at two electrodes of a differential capacitor are varied in such a way that the voltage excited on the third, common electrode, becomes zero, leading to a linear relationship between transducer voltages and displacement.
The object of the invention is to combine the constructional advantages of the simple capacitive system with the measuring advantages of the differential capacitive system in one length measuring-system.
According to the invention, this is achieved in that a measuring capacitor, whose capacitance is linearly modified by the displacement to be measured, is connected to a reference a.c. voltage, and a reference capacitor with the same dielectric is connected to an alternating voltage of the same frequency and opposite phase, called the measuring a.c. voltage, and that the amplitude of the measuring a.c. voltage can be varied by electronic means in such a way that the a.c. voltage induced on an electrode which is common to both capacitors becomes zero.
The advantage of said measure is that the transducer of a length-measuring system according to the invention is quite smaller than in the case of a differential capacitive system. This is particularly the case if the capacitance change of the measuring capacitor is brought about by a grounded screen inserted between both electrodes. Such an arrangement is very advantageous in the case of a transducer according to the invention, because it reacts with relatively limited sensitivity to guidance errors and is also advantageous for the electrical connections. As opposed to this, a differential capacitive transducer of known construction has a more complicated design because in this case the field lines must be intersected at two locations and not one. Furthermore, in the case of a measuring transducer according to the invention, the purposively incorporated pre-amplifier can be kept very simple and consequently very small, because it only serves as an impedance transformer in order to pass a corrective signal varying around zero to the remaining electronics. Thus, the input capacitance of this pre-amplifier has no significant influence on the measuring and reference capacitances, so that very small values of less than one picofarad are possible for the latter.
An embodiment of the length-measuring system according to the invention is described in greater detail and by way of example only with reference to the accompanying drawings wherein:
FIG. 1 shows in section the constructive principle of a measuring transducer according to the invention.
FIG. 2 the block diagram of the processing electronics embodied in this invention.
FIG. 3 the voltages in the electronics shown in FIG. 2 as a function of time.
As can be seen in FIG. 1, cylindrical electrodes 1 and 3 form a measuring capacitor Cm, whilst cylindrical electrodes 2 and 3 form a reference capacitor Cr. Whereas, reference capacitor Cr remains constant, the measuring capacitor Cm is linearly modified by the displacement X to be measured. This modification can be obtained in various ways, e.g. by a relative displacement of electrodes 1 and 3 or, as shown in FIG. 1, by inserting a screen 4, corresponding to the displacement X to be measured and which therefore brings about a capacitance change proportional to this displacement. The latter method has two advantages: firstly the thus constructed transducer is substantially insensitive to guidance errors of the screen, particularly with a cylindrical construction as in FIG. 1, and secondly the electrical connections to electrodes 1 and 3 are fixed, whilst screen 4 as the only movable part must be at the noncritical zero potential by means of a contact or movable line 5, whereby the latter may be a restoring spring.
Electrode 1 is excited by a constant a.c. voltage vr, called the reference a.c. voltage, and electrode 2 by a variable a.c. voltage vm, called the measuring a.c. voltage. The measuring a.c. voltage vm is varied by the electronic means described hereafter in such a way that the a.c. voltage vo induced on the common electrode 3 becomes zero. In this case, the sum of the capacitive currents flowing from electrode 3 is equal to zero, i.e.:
icm +icr =0,
so that in the case of a.c. voltages of the same type and frequency:
vr ·cm +vm ·cr =0
vm =-cm /cr ·vr
Thus, vm is in a linear relationship to the displacement X because the measuring capacitance Cm is proportional to the displacement X. In addition, changes to the dielectric constant have no influence if Cm and Cr have the same dielectric, e.g. air.
The described arrangement of the electrodes also permits in simple manner the calibration of the transducer sensitivity by adjusting the reference capacitance Cr, e.g. by means of a set screw 6, thus changing the ratio of the voltage change to the length change. This gives the possibility of alternately connecting different transducers to a common electronic processing unit without the need to recalibrate each time.
In order to ensure a completely satisfactory operation of a length-measuring system according to the invention, the voltage vo induced on a common electrode must naturally only be generated by the voltages vr and vm acting through the capacitors Cm and Cr. This means that the line which transmits vo from the transducer to the remaining electronics must be carefully screened from the lines carrying the exciting voltages vr and vm. A better and cheaper solution is provided by placing a simple impedance transformer 10 in the transducer. The construction of such an impedance transformer is known and does not present any particular problems because the amplification factor is not critical. However, even with low output impedance of the impedance transformer 10, couplings of the exciting a.c. voltages vr and vm can have an effect on the voltage vo' on the output side, which can lead to disturbing phase errors in the case of sinusoidal voltages.
However, if the exciting voltages vr and vm are square-wave voltages, the undesired couplings and time-lags appear as transients. These transients are produced by the flanks of the square waves and decay so that after a given settling time their disturbing action becomes negligible. Thus, it is sufficient to take no account of the excited voltage vo during this transient time.
This is for example achieved by the electronics shown in FIG. 2. The voltage vo' of impedance transformer 10 is applied to the input amplifier 14 which amplifies the signal to a level suitable for further processing. It is advantageous to construct input amplifier 14 as a current-voltage transformer so as to reduce the impedance on the input side substantially to zero. Thus, the transients on the line are suppressed and this eliminates any associated fault due to possible feedback through the impedance transformer 10 to its input.
An oscillator 11 produces a square wave vosc whose flanks define times t01, t02, t11 and t12 in the time-voltage diagram of FIG. 3. Square wave vosc is coupled to the frequency divider 13 so that a square wave with half the frequency and slightly delayed with respect to vosc appears at the output of this frequency divider or flip-flop. This square wave is used as the reference a.c. voltage vr, with the assumption that its amplitude is constant. This is ensured by selecting a logic, whose output level is clearly defined, e.g. CMOS. The measuring a.c. voltage vm is produced by alternatively switching between a subsequently defined measuring d.c. voltage Vm and a constant potential, in this case ground. The electronic single pole double throw switch 18 is controlled by the reference a.c. voltage vr and is wired up in such a way that the so generated measuring a.c. voltage vm has a phase opposed to the reference a.c. voltage vr, i.e. it is reversed. FIG. 3 shows the phase relationships between the oscillator voltage vosc and the two voltages vr and vm. The flanks of the reference a.c. voltage vr are representated by times t00, t10 and t20. The flanks of the measuring a.c. voltage vm approximately coincide with these times. If the two a.c. voltages vr and vm were perfect square-wave voltages with a phase displacement of precisely 180° and if there were no couplings, the a.c. voltage vo obtained on the common electrode 3 (FIG. 1) could be set precisely to zero by suitable regulation. As was shown herein before, the measuring a.c. voltage vm and consequently the measuring d.c. voltage Vm would then have a linear relationship to the displacement X to be measured. As such ideals conditions cannot be achieved, transients after the switching times t00, t10 and t20 appear on the induced a.c. voltage vo. A transient suppressor 15 is inserted between input amplifier 14 and synchronous demodulator 16 to suppress these transients. In the simplest case, this transient suppressor is an electronic switch, controlled by the voltage vosc in such a way that the switch is opened at the time t11, just before the appearance of the transient (e.g. t02 in FIG. 3), until said transient decays. Since, due to the time lag element 12 the voltage vosc assumes the level corresponding to the open state just before the flanks of voltages vr and vm and maintain this level for a sufficient time, a signal free from said transients is supplied to the synchronous demodulator 16, which has the advantage, as in the ideal case, that the signal is set by regulating means to the theoretically correct zero value. In the present example, said regulating means comprises a simple integrator 17 which is connected behind the demodulator 16. If the demodulated signal differs from zero, the output voltage Vm of the integrator will change as a function of the amplitude and polarity of the demodulated signal. As said output voltage is the measuring d.c. voltage Vm, the measuring a.c. voltage vm varies correspondingly until the voltage at the integrator input reaches zero. The thus obtained measuring d.c. voltage is therefore in a linear relationship to the displacement X and is consequently usable as the measuring d.c. voltage of said displacement X.
The displacement X can now be displayed by means of an analogue or digital voltmeter applied to the measuring d.c. voltage Vm or can undergo further processing.
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|U.S. Classification||324/662, 324/688, 340/870.37|
|International Classification||G01D5/24, G01D5/241, G01B7/00|